(1) Field of the Invention
This invention relates to the processing of nitride surfaces during the production of semiconductor devices. More particularly, the invention relates to a method of conditioning a semiconductor wafer including a deposited silicon nitride layer for subsequent processing steps. It is specifically useful in conditioning the surface of a nitride layer for subsequent application of photoresistive materials used for enabling the selective etching away of portions of the nitride layer.
The invention is particularly useful in fabricating metal-nitride-oxide silicon (MNOS) and silicon gate (SG) semiconductor devices. During fabrication of both types of devices, silicon nitride layers are deposited on a semiconductor wafer at some point at the wafer processing sequence. For various reasons, later in the processing sequence it becomes necessary to remove selected portions of the nitride layer by a masking and etching operation. For example, in a typical SG process a semiconductor wafer substrate is provided first with a thin oxide layer followed by a nitride layer; and then the nitride layer is selectively removed only in the field regions of the substrate so that an ion implant of the field region through the oxide layer can be performed. Later in this SG processing sequence the nitride layer and underlying oxide layer are removed from the gate region of the substrate so that a gate oxide can be grown on the substrate and the gate region implanted through the gate oxide.
In a typical MNOS process a silicon nitride layer is deposited over a gate oxide layer grown on a semiconductor substrate. Later, the nitride layer and the gate oxide layer are removed from previously defined contact apertures in a field oxide region so that metal can be deposited to contact the semiconductor substrate wherein source, drain, and other contact regions have been formed earlier in the process. Removal of selected portions of grown or deposited layers on a semiconductor wafer is typically accomplished by a photoresist-wet chemical etching procedure. This procedure requires good adherence of the final photoresist etchant mask to the underlying layer during etching to produce high resolution, well defined and controlled topology in the resultant structure and consistent high yields at these important steps in the wafer processing procedure.
(2) Description of the Prior Art
A standard well-known technique for producing a high quality etchant mask pattern on a silicon nitride layer involves the steps of (1) forming a thin masking oxide layer on the nitride surface, (2) densifying the oxide layer, (3) employing standard photomask photoresist techniques to form a photoresist etchant mask pattern on the masking oxide layer, and (4) etching the oxide layer, typically using buffered hydrofluoric acid, to produce an oxide etchant mask pattern on the surface of the nitride layer. Then, an etching solution which attacks only the nitride and not the oxide layer is employed to remove the nitride layer in exposed areas. This prior art method produces adequate results. Nevertheless, additional handling and the increased processing time appreciably raises manufacturing costs. For example, at least 1/2 hour in a chemical vapor deposition reactor is required to deposit on a silicon nitride layer a masking oxide layer that is sufficiently thick to accomplish the desired result. Further, at least an additional hour of processing time is required to densify the masking oxide for it to function adequately as an etchant mask. Finally, if an oxide layer under the nitride layer must also be removed as in several examples given above, a separate oxide etch must be employed after the selective nitride etch. These additional processing steps also complicate the overall process.
The reason for the use of the masking oxide is that photoresistive materials adhere very poorly when applied directly to a nitride surface. Poor photoresist adhesion results in loss of the required high resolution photoresist etchant mask pattern while the nitride layer is subjected to the etching solution. Consequently, the ability to produce sharply defined and closely controlled nitride layer topology is seriously degraded.
It has previously been proposed to apply a heated solution of trichlorophenylsilane to a nitride surface to improve the adhesion of photoresist applied thereafter to the coated nitride surface. (See Michael R. Gulett application. "Improved Semiconductor Processing", U.S. Ser. No. 668,167, filed Mar. 18, 1976). Other organosilane solutions such as hexalkyldisilazane solutions and, in particular, hexamethyldisilazane, have also been found to improve somewhat the adhesion of photoresist to silicon nitride. However, the successful use of these organosilane solutions to aid in photoresist adhesion to silicon nitride requires very rigorous control measures during wafer processing to achieve consistent acceptable results. Such rigorous quality control is difficult to achieve in a high volume manufacturing environment in which consistent high yields at every step in the process are required to maximize overall net process yields and thereby minimize production costs of semiconductor devices.